v°/EPA
                                United States
                                Environmental Protection
                                Agency
                                 Environmental Sciences Research
                                 Laboratory
                                 Research Triangle Park NC 277
                                Research and Development
                                EPA-600/S4-81-030 June 1981
Project  Summary
                                Dependence  of Nephelometer
                                Scattering Coefficients on
                                Relative  Humidity:  Evolution
                                of Aerosol Bursts
                                George W. Griffing
                                  Observations  on  the  temporal
                                dependence of the nephelometer scat-
                                tering coefficient on  the ambient
                                relative humidity are presented and
                                discussed. The observations are repre-
                                sentative of the temporal dependence
                                of the scattering coefficient when the
                                weather at the Research Triangle Park
                                is dominated  by  an  anticyclonic
                                weather system. By taking simultan-
                                eous nephelometer scattering coeffi-
                                cient observations at two different
                                relative humidities, it was possible to
                                conclude that with stable atmospheric
                                conditions:

                                  •  In general, the scattering coeffi-
                                    cient increases from sundown to
                                    sunup due to aerosol growth and
                                    an increasing trend of the aerosol
                                    number density;

                                  •  In general,  the relatively rapid
                                    increase  and  .subsequent de-
                                    crease of the scattering coeffi-
                                    cient during a 2 to 3 hour period
                                    after sunup is due to a relatively
                                    rapid aerosol growth and shrink-
                                    age, and  a  relatively  rapid
                                    increase  and decrease  of the
                                    aerosol number density.

                                  The latter behavior of the scattering
                                coefficient was  called  an  aerosol
                                burst. The relationship between an
                                aerosol burst, fumigation, and early
                                morning visibility deterioration is also
                                discussed.
                                  This Project Summary was develop-
                                ed by EPA's Environmental Sciences
                                Research Laboratory, Research Tri-
                                angle Park, NC, to announce key find-
                                ings of the research project that is fully
                                documented in a separate report of the
                                same title (see Project Report ordering
                                information at back).


                                Introduction
                                  According  to  Section 169A of the
                                Clean Air Act Amendment of 1977
                                (Federal  Register, 1978), the Environ-
                                mental  Protection Agency  has  the
                                responsibility of  preventing visibility
                                degradation  of designated Class  I
                                Federal  areas. There can be various
                                causes for a degradation of the visibility.
                                One cause for visibility degradation is
                                an increase of the  aerosol number
                                density  which might  be reduced by
                                control of certain anthropogenic emis-
                                sions. Another cause for visibility
                                degradation might beduetoan increase
                                of the relative humidity. With the possi-
                                ble exception of the  control  of the
                                emission of  moisture from cooling
                                towers or similar structures, the control
                                of the relative humidity would not be
                                feasible  for  prevention  of visibility
                                degradation.
                                  For various Environmental Protection
                                Agency  visibility related studies, a
                                nephelometer  has  been   used. To
                                interpret the nephelometer data, an
                                understanding of the possible influence
                                of the relative humidity is essential. If
                                                                  > US. GOVERNMENT PRINTING OFFICE 1M1-757-012/7135

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   density increased. However, the scat-
   tering  coefficient  could  also  have
   increased because the relative humidity
   increased, which  could  result in an
  .increase in the size of the aerosols. A
   decrease  of  the  visibility would be
   expected for  either  of these possible
   reasons for an increase of the scattering
   coefficient.
     Since December 1978, studies have
   been conducted at the Research Tri-
   angle Park (RTP) on the dependence of
   the nephelometer scattering coefficient
   on  the ambient  relative  humidity.  In
   order to extract more information from
   these  observations, simultaneous
   nephelometer scattering  coefficient
   observations  are taken at a  different
   (usually much smaller) relative humid-
   ity. Except for brief periods for calibra-
   tion, maintenance,  and repair,  the
   observations  have  been  taken
   continuously.
     In these studies, it  has been found
   that the  most  satisfactory  humidity
   sensor was the saturated salt (lilthium
   chloride) humidity sensor. The cooled
   mirror  type  humidity sensor was not
   found to be reliable because of contami-
   nation  of the mirror  by atmospheric
   aerosols. At times, the mirror would get
   contaminated within a few minutes. At
   other times, it would be a day before the
   mirror got contaminated. The saturated
   salt humidity sensor was very much less
   sensitive  to  contamination.   Usually,
   cleaning of the sensor was not neces-
   sary before  a couple  of months  of
   operation.
     The intake for atmospheric  air to the
   nephelometers were 50 m apart and 6
   m above ground level. Special attention
   was given to the problem of obtaining
        relative humidities which were charac-
        teristic of the nephelometer scattering
        chamber.  Wind  observations  were
        taken at 6 m. Also, solar radiation data
        were taken at 6 m. Both wind and solar
        observations are useful in an  interpre-
        tation of the observations.
        Conclusions
          By operating one nephelometer at
        ambient relative humidity and another
        nephelometer at  a smaller relative
humidity,  it  is possible  to  obtain an
insight into some of the temporal vari-
ations  of  the physiochemical param-
eters  characterizing the  atmospheric
aerosols. Thus, by comparing the scat-
tering  coefficient  profiles of the two
nephelometers it is possible to deduce
what effect that relative humidity had
on the scattering coefficient. In addition,
it  is possible to gain  insight into the
meteorological processes involved. The
important  results of the study were
enumerated earlier.
          The author of this Project Summary was George W. Gritting who is with the
           Environmental Sciences Research Laboratory, EPA, Research Triangle Park
           NC 27711.
          K. L.  Demerjian is the EPA Project Officer (see below).
          The complete report, entitled "Dependence of Nephelometer Scattering Coeffi-
           cients on Relative  Humidity:  Evolution of Aerosol Bursts,"  (Order  No.
           PB  81-198 293; Cost: $6.50, subject to change) will be available only from:
                 National Technical Information Service
                 5285 Port Royal Road
                 Springfield, VA 22161
                 Telephone: 703-487-4650
          The EPA Project Officer can be contacted at:
                 Environmental Sciences Research Laboratory
                 U.S. Environmental Protection Agency
                 Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
                Postage and
                Fees Paid
                Environmental
                Protection
                Agency
                EPA 335
Official Business
Penalty for Private Use $300
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